I just found out that Effective Perl Programming is available in digital formats through eBooks.com. They have a PDF version and an ePub version, each for $US31.99, which, sadly, is more than the hard-copy price of $US26.39 on Amazon.com and the Kindle price of $US17.59.
They specifically list that these formats target Adobe Digital Editions, so I don’t know if they’ve checked them on any other readers or have used special features.
InformIT also has an “eBook” version, which I think is just a PDF, for $US28.79, but also has a “Book + eBook Bundle” for $US47.19. They might offer ePub in the future.
If anyone buys either the PDF or ePub versions from eBooks or InformIT, let us know what you think of them, how they look, and which reader you use.